Research concerning condensation between an epoxide or an epoxide derivative and a cyclic anhydride has been summarized by LUSTON and VASS (Advances in Polymer Sciences 1984, Vol. 56, p. 91 et seq. or by ISHII and SAKAI (Ring opening polymerisation, p. 13 et seq. published by K. C. FRISCH and S. REEGEN, MARCEL DEKKER 1969).
An examination of the cited works shows that one of the main problems posed by this type of condensation is the homopolymerization of the epoxide, resulting in either a sequent polyether-polyester or a polymer mixture, particularly when LEWIS acids (TiCl.sub.4, BF.sub.3, etc.) are used as condensation catalysts. In order to overcome this drawback. The prior art suggests utilizing anionic or coordination catalysts.
FISHER (Journal of Polymer Science 1960, Vol. 44, p. 155 et seq.) has shown that the use of a tertiary amine as a condensation catalyst of an anhydride on an epoxide results in an alternate condensation.
However, this type of catalyst is ineffective in the case of maleic anhydride, probably because of the complex side reactions with the amines at the level of the double maleic bone. Other types of anionic catalysts, such as alkali metal salts, or tetra-alkylammonium salts have also been utilized. For example, WADILL. MILLIGAN and PEPPEL (Industrial and Engineering Chemistry, Product Research and Development 1964, Vol. 3, Part 1, p. 53 et seq.) describe the use of lithium chloride in the presence of protonic materials at 150.degree. C. These authors suggest that the homopolymerization of epoxide is part of their process. As an example of a coordination catalyst. INOUE et al. mention by INOUE et al. (Makromolekulare Chemie 1969, Vol. 126, p. 250 et seq.), mention dialkylzinc, but they note that this type of catalyst can actually only be applied to phthalic anhydride.
U.S. Pat. No. 4,565,845 describes the use of a catalytic system comprising an aluminium porphyrin to obtain a polyester with good alternation between the unit from the anhydride and that from the epoxide.
Other catalysts, based on transition metals, have also been previously described. Thus, FISCHER (cited above) observes a partial homopolymerization of the glycidic epoxide during its polycondensation with the phthalic anhydride in the presence of tetrabutyl titanate.
U.S. Pat. No. 3,546,176 claims the use of tetrabutyl titanate for producing unsaturated polyesters from anhydrides of unsaturated acids and epoxides. But, as confirmed by a test carried out by the applicants and described hereafter, this catalyst does not obtain good alternation of the units from the anhydride and from the epoxide.